We are presently engaged in a research project, supported by the National Institute of Health, having as its goal, determination of accurate and precise enthalpies of formation of molecules of medical or biochemical importance. The method depends on hydrogenation of an unsaturated reactant of unknown heat of formation to give a saturated product whose heat of formation is known. The heat of formation of the unknown is calculated by simple Hess' law addition. Work is presently directed to determination of heats of formation of the numerous mono- and polyunsaturated fatty acids, alcohols, esters and glycerides of nutritional importance by hydrogenating them to their relatively common and better-known saturated products such as stearic acid and methyl stearate.